Systems for delivering respiratory gas to patients include apparatus which serves as the source of air, oxygen and sometimes other respiratory gases, in appropriate proportion and at an appropriate positive pressure. The gas is delivered to the patient through a delivery conduit to what is sometimes called a "patient connection". The patient connection may be a mask or an intubation set, in incubator or even a tent. Particularly when the purpose of the respiratory system includes forced or paced ventilation of the patient, the system will include an exhaust conduit which extends from the patient connection to a point of negative gage pressure.
In many cases it is required that the gas be delivered to the patient connection at an elevated temperature near body temperature and that the gas be humid to the point of saturation or nearly so. Heating and humidifying are readily accomplished. However, to accomplish them in a way that minimizes discomfort and danger to the patient is seldom easy. The problem is twofold. The delivered gas must not exceed safe temperature and water, humidifying water and condensate, must not reach the patient. If humidification of the gas is accomplished at the supply apparatus or in the supply conduit at an appreciable distance from the patient connection, an excessive amount of condensate will form in the delivery conduit as the gas passes along the conduit and is cooled. If that condensate reaches the patient's mask or intubation set, the patient, an infant or comatose patient for example, may be drowned.
Temperature measurement and control is accomplished automatically with relative ease. Heating is done electrically and is readily controlled electronically. Humidification is less convenient. The conventional system employs a "bubbler", a container of water connected in the gas delivery conduit such that the gas must flow through the water. Gas from the source is conducted to a point in the bubbler below the level of the water. There it emerges into the water and bubbles up to the ullage space above the water. Still under pressure, it flows to the patient connection. Heating is accomplished at the humidifier or upstream from it for to heat it downstream would reduce the degree of humidification. Moving the humidifier downstream reduces the opportunity for condensate to form but it increases the danger that a spill in the humidifier will reach and drown the patient. In practice, the distance from the gas source to patient connection differs from case to case, patients must be free to move or be moved and, for additional reasons, the conduits and humidifier and heater are often assembled into jury rigged, temporary flow circuits. The danger that a humidifier or water trap will be dislodged from its mounting and upset to spill water to the patient side of the device is a continuing danger.
The disadvantages of the bubbler humidifier do not end with their potential for drowning patients. Because the water they use is included in the flowpath, sterile water must be used. Moreover they are noisy. The constant bubbling sound all through an otherwise quiet night can interfere with sleep and be irritating. In the case of a sedated patient, the constant sound can be stressful.